First and foremost, you cannot lose this battle, as you have a permanent Auto-Life. Unless you purposefully go out of your way to petrify your entire team themselves, you will be able to win no matter what. The first portion of the battle will have Yuna summoning all of her Aeons, then Yu Yevon possessing them, requiring you to defeat each one. Now, all of the Aeons will have the stats they had when they were in your control, so you should have a good idea of what they can do. The Pagodas will be in this fight, although they can now be reached with melee attacks. They will charge up each Aeon’s Overdrive, but it doesn’t really matter.
You can summon the Aeons in any order you want and upon defeating the last one, Yu Yevon will finally appear himself. On each of his turns, he will either do nothing or use Gravija on the party, taking away 75% of their current health. Should you attack him, then he will counter with Curaga, healing 9,999 HP. Once you uses Curaga seven times, he will use Osmose, then Ultima on the entire party, before resorting to the beginning of his script. It should be noted that his Gravija will also hurt him, so you could literally do nothing in the battle and have him reduce his HP to a lower enough amount before delivering the final blow.
The above is the strategy you will want to use if you don’t have access to Break Damage Limit, which is on all of the Celestial Weapons, by the way. If you happen to have a Candle of Life item on you, which is a common steal from the [Fallen Monks] in Zanarkand, having someone like Rikku Use one on Yu Yevon will cast Doom. Unlike a lot of harder enemies in the game, Yu Yevon will succumb to Doom in only three turns, making it a lot easier than having to deal with his Curaga spam. You could also do something as simple as cast Reflect on him, causing Curaga to be knocked back to your party.